System and method for managing the selection of ghost channels for mitigating polarization hole burning

TECHNICAL FIELD

p-0002This invention relates generally to the field of optical communication networks and more specifically to managing the selection of ghost channels for mitigating the effects of polarization hole burning.

BACKGROUND

p-0003A communication network includes paths of nodes that route packets through the network. Optical amplifiers perform an important function within these networks by amplifying an optical signal in order to increase the operational length of an optical network. In some configurations, the efficiency of an optical communication network may be compromised by an effect known as polarization hole burning.

p-0004Optical communication systems designed to operate over long distances may suffer from multiple polarization-dependent effects that reduce the operational efficiency of the system. Polarization hole burning (PHB) is one of these phenomena. PHB may seriously reduce the performance of rare-earth doped fiber optical amplifiers, such as an erbium doped fiber amplifier (EDFA), used to amplify signal strength within the communication system.

p-0005PHB occurs when a strong, polarized optical signal is launched into an EDFA. This strong signal can cause anisotropic saturation of the amplifier. This saturation effect, which is related to the population inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same state of polarization (SOP) as the saturating signal. Thus, PHB causes a signal having a SOP orthogonal to the saturating signal to have a gain greater than that of the saturating signal.

p-0006As a result, amplified spontaneous emission (ASE) noise in the SOP orthogonal to the saturating signal may accumulate faster than in the SOP of the saturating signal. In a communication system utilizing a chain of EDFAs operating at or near saturation, ASE noise may accumulate at each amplifier stage. As the noise builds up over the course of the system, the signal-to-noise ratio (SNR) for a signal with a SOP orthogonal to the saturating signal may rise to unacceptable levels. The SNR in such cases can then cause errors in the received data stream. Accordingly, mitigating the effects of PHB in amplified optical systems is desirable.

p-0007One of the causes of the undesirable PHB effect is operating an EDFA in a way that leads to gain compression. Gain compression (“Cp”) is a measure of the difference of the amplifier's non-saturated gain (“Go,” or the gain when operating on a low power signal) and the amplifier's saturated operating gain (“G”). The operating gain, in decibels, can be measured by taking the difference between the saturated output power (“So”) and the input power of a saturating signal (“Si)”, as follows: <br /><i>G=So−Si. </i>

p-0008The corresponding gain compression may be calculated as the difference between the non-saturated gain and the saturated operating gain: <br /><i>Cp=Go−G. </i>

p-0009The gain in the SOP orthogonal to a saturating signal may be measured using a probe signal with an input signal orthogonal to the saturating signal by measuring the input power (“Pi”) and output power (“Po”) of the probe signal: <br /><i>Po−Pi=G+ΔG. </i>

p-0010The “ΔG” in the above formula represents the amount of PHB in the SOP orthogonal to the saturation signal. This is a result of operating the amplifier with a saturating signal. As gain compression of an amplifier increases, so does the amount and effect of PHB. For instance, a single EDFA operating at a gain compression of about 3 dB may produce a PHB of about 0.08 dB. However, when that EDFA operates in a more saturated condition, with Cp=9-10 dB, the PHB may rise to about 0.2 dB.

p-0011The degree of PHB may also be affected by other factors, such as the degree of polarization of the saturating signal. If a signal's SOP varies over time, the effects of PHB may be reduced.

p-0012While the degree of PHB may be small for a single EDFA, these effects may be seriously compounded in communication systems that chain together a series of EDFAs. A number of arrangements have been proposed for reducing the effects of PHB in optical communication systems. However, such arrangements continue to suffer from drawbacks such as an inability to deal with arbitrary channel loading, expense and difficulty of implementation, and the innate stability characteristics of rare-earth doped fiber amplifiers.

SUMMARY OF THE DISCLOSURE

p-0013In accordance with the present invention, disadvantages and problems associated with previous techniques for mitigating the effects of polarization hole burning in optical amplifiers may be reduced or eliminated.

p-0014According to one embodiment of the present invention, a system is provided for managing the selection of ghost channels in an optical communication system. The system includes components configured to collect one or more first data values indicating the validity of an optical communication channel within a first degree of a node in the optical communication system, collect one or more second data values indicating the optical power level of the optical communication channel, transmit the first and second data values to a second degree of the node, receive the first and second data values at the first degree, and aggregate the first and second data values for the first degree and the second degree at the first degree.

p-0015According to another embodiment of the present invention, a method is provided for managing the selection of ghost channels in an optical communication system. The method includes collecting one or more first data values indicating the validity of an optical communication channel within a first degree of a node in the optical communication system, collecting one or more second data values indicating the optical power level of the optical communication channel, transmitting the first and second data values to a second degree of the node, receive the first and second data values at the first degree, and aggregating the first and second data values for the first degree and the second degree at the first degree.

p-0016Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that creating ghost channels around a saturating signal in an optical communication network increases the signal to noise ratios of those signals with adjacent ghost channels. In some embodiments, the creation of one or more ghost channel(s) on one side or both sides of an optical signal may provide a low-cost implementation for polarization hole burning mitigation.

p-0017Certain embodiments of the invention may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0018For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

p-0019<figref idrefs="DRAWINGS">FIG. 1</figref> illustrates an optical communication network system, in accordance with certain embodiments of the present disclosure;

p-0020<figref idrefs="DRAWINGS">FIG. 2</figref> illustrates a node with a plurality of degrees, in accordance with certain embodiments of the present disclosure;

p-0021<figref idrefs="DRAWINGS">FIG. 3</figref> illustrates a graph of a traffic channel surrounded on either side by ghost channels, in accordance with certain embodiments of the present disclosure;

p-0022<figref idrefs="DRAWINGS">FIG. 4</figref> illustrates an optical amplification scheme for generating ghost channels, in accordance with certain embodiments of the present disclosure;

p-0023<figref idrefs="DRAWINGS">FIG. 5</figref> is a flowchart illustrating one embodiment of a method of mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure;

p-0024<figref idrefs="DRAWINGS">FIG. 6</figref> illustrates a table for storing information regarding the validity of a channel incoming to a degree of node, in accordance with certain embodiments of the present disclosure;

p-0025<figref idrefs="DRAWINGS">FIG. 7</figref> illustrates a table for storing information regarding the validity and signal strength of a channel incoming to a particular degree of node, in accordance with certain embodiments of the present disclosure;

p-0026<figref idrefs="DRAWINGS">FIG. 8</figref> illustrates a series of tables representing the consolidated information received from other degrees of node, in accordance with certain embodiments of the present disclosure;

p-0027<figref idrefs="DRAWINGS">FIG. 9</figref> illustrates a table for storing information regarding the current ghost channel load for degrees in node, in accordance with certain embodiments of the present disclosure;

p-0028<figref idrefs="DRAWINGS">FIG. 10</figref> is a flowchart illustrating one embodiment of a method of managing the selection of ghost channels in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure;

p-0029<figref idrefs="DRAWINGS">FIG. 11</figref> is a flowchart illustrating one embodiment of a method of selecting ghost channels for use in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure;

p-0030<figref idrefs="DRAWINGS">FIG. 12</figref> is a flowchart illustrating one embodiment of a method of selecting the degree of node from which to select the appropriate ghost channels for mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure; and

p-0031<figref idrefs="DRAWINGS">FIG. 13</figref> is a flowchart illustrating one embodiment of a method of optimizing a ghost channel selection routine in order to mitigate the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

p-0032Embodiments of the present invention and its advantages are best understood by referring to <figref idrefs="DRAWINGS">FIGS. 1 through 13</figref> of the drawings, like numerals being used for like and corresponding parts of the various drawings.

p-0033<figref idrefs="DRAWINGS">FIG. 1</figref> illustrates an optical communication network system, in accordance with certain embodiments of the present disclosure. Optical network system <b>10</b> includes components such as network nodes <b>22</b>. In general, a network node <b>22</b> may include any suitable arrangement of components operable to perform the operations of the network node. As an example, a network node may include logic, an interface, memory, other component, or any suitable combination of the preceding. “Logic” may refer to hardware, software, other logic, or any suitable combination of the preceding. Certain logic may manage the operation of a device, and may comprise, for example, a processor. “Processor” may refer to any suitable device operable to execute instructions and manipulate data to perform operations.

p-0034“Interface” may refer to logic of a network node operable to receive input for the network node, send output from the network node, perform suitable processing of the input or output or both, or any combination of the preceding, and may comprise one or more ports, conversion software, or both.

p-0035“Memory” may refer to logic operable to store and facilitate retrieval of information, and may comprise Random Access Memory (RAM), Read Only Memory (ROM), a magnetic drive, a disk drive, a Compact Disk (CD) drive, a Digital Video Disk (DVD) drive, removable media storage, any other suitable data storage medium, or a combination of any of the preceding.

p-0036Network system <b>10</b> communicates information through signals, such as an optical signal. As an example, an optical signal may have a frequency of approximately 1550 nanometers and a data rate of 10, 20, 40, or over 40 gigabits per second.

p-0037According to the illustrated embodiment, network system <b>10</b> may include one or more networks. A network may include nodes <b>22</b> coupled by fibers <b>26</b> in a mesh topology as shown in <figref idrefs="DRAWINGS">FIG. 1</figref> or any other suitable topology, such as a liner or ring topology.

p-0038The components of network system <b>10</b>, coupled together by the optical fibers <b>26</b>, may include one or more reconfigurable optical add/drop multiplexers (ROADM), one or more amplifiers, and one or more splitters, as described in more detail below with reference to <figref idrefs="DRAWINGS">FIG. 2</figref>. Network system <b>10</b> may be used in any optical communication network, or any other suitable network or combination of networks. Optical fibers <b>26</b> comprise any suitable type of fiber, such as a Single-Mode Fiber (SMF), Enhanced Large Effective Area Fiber (E-LEAF), or TRUEWAVE® Reduced Slope (TW-RS) fiber.

p-0039In a given topology, each node <b>22</b> will have an associated number of “degrees.” The number of degrees of node <b>22</b> may be defined to be the number of links incident to node <b>22</b>. In the illustrated embodiment, a portion of a mesh topology consisting of four nodes <b>22</b> is shown. Each node <b>22</b> has four degrees: three links to the other nodes <b>22</b> of network system <b>10</b> and a link to the remaining portion of network system <b>10</b>. The number of degrees may be any number, depending on the particular topology and implementation chosen.

p-0040In some embodiments, network system <b>10</b> may be designed to assign each incoming signal to a particular “channel,” or carrier wavelength. The number of channels and the wavelengths assigned may vary depending on the chosen implementation. As an illustrative example, network system <b>10</b> may carry 88 channels in the 1550 nm wavelength band, with a channel separation of 50 GHz (˜0.4 nm). That is, network system <b>10</b> may potentially communicate information on carrier wavelengths between 1528.77 nm (196.1 THz) and 1563.45 nm (191.75 THz). In some embodiments, network system <b>10</b> may include some means of dynamically allocating incoming signals to various wavelengths, depending on the design needs, such that none, some, or all channels are in use at one time.

p-0041The process of communicating information over multiple channels of a single optical path is referred to in optics as wavelength division multiplexing (WDM). Dense wavelength division multiplexing (DWDM) refers to the multiplexing of a larger (denser) number of wavelengths, usually greater than forty, onto a fiber. WDM, DWDM, or other multi-wavelength transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM or DWDM, the bandwidth in networks would be limited to the bit rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Referring back to <figref idrefs="DRAWINGS">FIG. 1</figref>, network system <b>10</b> is operable to transmit disparate channels using WDM, DWDM, or some other suitable multi-channel multiplexing technique, and to amplify the multiplexed wavelengths <b>40</b>.

p-0042<figref idrefs="DRAWINGS">FIG. 2</figref> illustrates a node <b>22</b> with a plurality of degrees <b>200</b>, in accordance with certain embodiments of the present disclosure. In the illustration, four degrees <b>200</b> are shown, but more or fewer degrees <b>200</b> may be present in a given configuration. Each degree <b>200</b> of node <b>22</b> may include splitter <b>202</b>, wavelength selective switch (WSS) <b>204</b>, optical channel monitor (OCM) <b>208</b>, and one or more amplifier(s) <b>206</b>. In operation, degree <b>200</b> of node receives the multiplexed wavelengths <b>40</b> from another degree <b>200</b> of node <b>22</b>, another node <b>22</b>, or some other portion of network system <b>10</b>. The channels comprising the multiplexed wavelengths <b>40</b> incoming to degree <b>200</b> of node <b>22</b> may be added, dropped, and/or amplified before exiting degree <b>200</b> of node <b>22</b>.

p-0043Amplifier <b>206</b> may be used to amplify the multiplexed wavelengths <b>40</b>. Amplifier <b>206</b> may be positioned before and/or after certain lengths of fiber <b>26</b>. Amplifier <b>206</b> may comprise an optical repeater that amplifies the optical signal. This amplification may be performed without opto-electrical or electro-optical conversion. In some embodiments, amplifier <b>206</b> may comprise an optical fiber doped with a rare-earth element. When a signal passes through the fiber, external energy is applied to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, amplifier <b>206</b> may comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifier <b>206</b> may be used. In the illustrated embodiment, each degree <b>200</b> of node <b>22</b> has a plurality of amplifiers <b>206</b> on an amplifier card <b>212</b>. Amplifier card <b>212</b> may, in some embodiments, also be configured to gather information about multiplexed wavelengths <b>40</b>, as described in more detail below with respect to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>. Although the figure shows each degree <b>200</b> with its own amplifier card <b>212</b>, there may be a single amplifier card <b>212</b> for some or all degrees <b>200</b> of node <b>22</b> or multiple nodes <b>22</b>.

p-0044When the optical strength of the multiplexed wavelengths <b>40</b> reaches a certain point, amplifier <b>206</b> may reach its maximum linear response. Past this point, amplifier <b>206</b> may behave in a non-linear fashion (referred to as being “saturated”). At these levels, the multiplexed wavelengths <b>40</b> may be referred to as a “saturating signal.” When amplifier <b>206</b> is supplied with a saturating signal, it may experience greater degrees of certain negative effects such as polarization hole burning (PHB). PHB may act to create a difference in the amount of amplified spontaneous emission noise in the same SOP as the saturating signal and the amplified spontaneous emission noise in the SOP orthogonal to the saturating signal. This difference leads to an overall decrease in the signal-to-noise ratio. Additionally, the magnitude of the decrease is dependent on the polarization state of the saturating signal and thus varies over time. This can result in both a reduction of signal quality and a time-varying signal quality at later nodes <b>22</b>. The effects of PHB can be mitigated through the use of ghost channels generated from the Amplified Spontaneous Emission (ASE) noise present within network system <b>10</b>, as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 3-13</figref>. After amplification, if required, the multiplexed wavelengths <b>40</b> may then pass to splitter <b>202</b>.

p-0045Splitter <b>202</b> may include any device or component of a device which may be configured to reproduce the multiplexed wavelengths <b>40</b> (at greater or lesser magnitudes) before passing on multiple copies of the multiplexed wavelengths <b>40</b> to other components of node <b>22</b> or to other nodes <b>22</b> of network system <b>10</b>. One such copy of multiplexed wavelengths <b>40</b> may be passed to WSS <b>204</b>. WSS <b>204</b> may include any device or component of a device which may be configured to receive, combine, add, drop, and/or amplify the component channels of the incoming multiplexed wavelengths <b>40</b> transmitted from other degrees <b>200</b> of node <b>22</b> or other nodes <b>22</b> of network system <b>10</b>. In some embodiments, WSS <b>204</b> may be configured to generate ghost channels and/or amplify or attenuate previously generated ghost channels, as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 3-5</figref>.

p-0046OCM <b>208</b> may include any component or set of components operable to provide information on the optical power of the individual optical channels comprising the multiplexed wavelengths <b>40</b>. In some embodiments, OCM <b>208</b> may be an integrated part of switch card <b>214</b>, such as that found on the FUJITSU FLASHWAVE 7500 ROADM and FLASHWAVE 9500 ROADM. In other embodiments, OCM <b>208</b> may be a stand-alone component, or the functions of OCM <b>208</b> may be performed by WSS <b>204</b> or any other appropriately configured component of node <b>22</b>. In operation, OCM <b>208</b> may measure at least the optical power of the signal component channels in order to provide this power information to other components of node <b>22</b> for the appropriate generation, selection, management, and optimization of ghost channels for mitigating the effects of PHB. Switch card <b>214</b>, or a component of switch card <b>214</b>, may also be configured to provide the functions of splitter <b>202</b>, WSS <b>204</b>, OCM <b>208</b>, or any other necessary functions, such as the maintenance of information regarding the current ghost channel load of degree <b>200</b>, as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>.

p-0047<figref idrefs="DRAWINGS">FIG. 3</figref> illustrates a graph <b>300</b> of a traffic channel <b>304</b> surrounded on either side by ghost channels <b>302</b>, in accordance with certain embodiments of the present disclosure. In the illustrated embodiment, traffic channel <b>304</b> and ghost channels <b>302</b> are shown at specific wavelengths and specific amplitudes. These and other specific properties of traffic channel <b>304</b> and ghost channels <b>302</b> are intended for illustrative purposes only, and are in no way intended to limit the scope of the present disclosure.

p-0048In some embodiments, node <b>22</b> comprises certain modules configured to generate ghost channels <b>302</b> surrounding traffic channel <b>304</b>. These ghost channels <b>302</b> may operate to reduce the deleterious effects of PHB. A ghost channel <b>302</b> generally refers to the propagation of optical energy through a communication channel of network system <b>10</b> without transmitting any signal information. Ghost channels <b>302</b> may be treated like other channels in need of amplification and/or propagation, but do not carry signal information.

p-0049Ghost channels <b>302</b> have been amplified to have an optical strength at or near the optical strength of traffic channel <b>304</b> as described in more detail below with reference to <figref idrefs="DRAWINGS">FIG. 4</figref>. Traffic channel <b>304</b> is a channel within a multi-channel optical signal that is carrying information along its carrier wavelength at a given moment. Generation, selection, management, and optimization of the ghost channels are discussed in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 4-13</figref>.

p-0050In some embodiments, traffic channel <b>304</b> may have only a single ghost channel <b>302</b> on either side. In other embodiments, there may be two, four, or any number less than the current capacity of network system <b>10</b>. In some configurations, the use of higher numbers of ghost channels <b>302</b> may result in the possibility of feedback loops within network system <b>10</b>. A given implementation may balance the desire for increasing the number of ghost channels <b>302</b> (and the corresponding decrease in the effects of PHB) and the desire to avoid feedback.

p-0051A given channel may have one or more “neighbor” channels. Neighbor channels are defined generally to be those that are within a certain wavelength distance of a given channel. For instance, if network <b>10</b> has implemented a channel separation of 50 GHz (˜0.4 nm), then channels at 1544.92 nm and 1545.72 nm may be the neighbors of the channel at 1545.32 nm. In other embodiments, a neighbor channel may be defined to be within an optical bandwidth equivalent to two or more times the channel spacing bandwidth (e.g., 50 GHz in the illustrative example), depending on the system's design criteria (such as the fear of signal interference and the intrinsic properties of the chosen carrier wavelength). Neighbor channels may also be said to “surround” a signal channel.

p-0052The effects of PHB may be most severe in situations where a particular traffic channel <b>304</b> is isolated from other traffic channels <b>304</b>. If network system <b>10</b> is operating at full capacity, with all channels carrying information at the same time, then each operating channel is surrounded by other operating channels. In such a case, the surrounding channels function to mitigate the effects of PHB for any given channel. However, rarely does network system <b>10</b> operate at such a full capacity. Often only a percentage of the communication channels carry information at any given time.

p-0053Some prior solutions have randomly., rotated the SOP of communication signals so that PHB effects will not be severe in any particular state of polarization. However, such solutions may be cost prohibitive to implement and, depending on the chosen implementation for rotating the SOP of the saturating signal, may not be able to successfully handle dynamic loading of channels across network system <b>10</b> (situations where the number and identity of information-carrying channels changes over time). In such situations, PHB effects may be mitigated by surrounding a communication channel with ghost channels.

p-0054In some embodiments, traffic channel <b>304</b> may be surrounded by ghost channels if it does not have another traffic channel <b>304</b> sufficiently nearby. This determination may be made in accordance with a set of predetermined rules. For instance, if, for a first optical signal, the next nearest optical signal is more than 20 channels away, then the first optical signal may require accompanying ghost channels in order to mitigate the effects of polarization hole burning. The more isolated a signal, the stronger the need for ghost channels. Below are example situations in which it might be desirable to produce ghost channels <b>302</b> for a given traffic channel <b>304</b> (denoted by λ_i). <ul><li id="ul0001-0001" num="0000"><ul><li id="ul0002-0001" num="0054">if . . . λ_iε{ch<b>1</b>, . . . , ch<b>22</b>} . . . and . . . λ_i−1&amp; λ_i+1∉{ch<b>1</b>, . . . , ch<b>23</b>} and N={1, . . . , 42},</li><li id="ul0002-0002" num="0055">λ_i−1, λ_i+1are channels adjacent to λ_iand N is the total number of channels propagating in a span.</li></ul></li></ul>

p-0055if . . . λ_iε{ch<b>23</b>, . . . , ch<b>44</b>} . . . and . . . λ_i−1&amp; λ_i+1∉{ch<b>22</b>, . . . , ch<b>44</b>}, and N={1, . . . , 42} <ul><li id="ul0003-0001" num="0000"><ul><li id="ul0004-0001" num="0057">λ_i−1, λ_i+1are channels adjacent to λ_i. and N is the total number of channels propagating in a span.</li><li id="ul0004-0002" num="0058">if . . . λ_i, λ_i+1ε{ch<b>1</b>, . . . , ch<b>22</b>} . . . and . . . λ_i−1&amp; λ_i+2∉{ch<b>1</b>, . . . , ch<b>23</b>}, and N={2 . . . , 42}</li><li id="ul0004-0003" num="0059">λ_i−1, λ_i+1are channels adjacent to λ_iand λ_i, λ_i+2are channels adjacent to λ_i+1. N is the total number of channels propagating in a span.</li><li id="ul0004-0004" num="0060">if . . . λ_i, λ_i+1ε{ch<b>23</b>, . . . , ch<b>44</b>} . . . and . . . λ_i−1&amp; λ_i+2∉{ch<b>22</b>, . . . , ch<b>44</b>}, and N={2 . . . , 42}</li><li id="ul0004-0005" num="0061">λ_i−1, λ_i+1are channels adjacent to λ_iand λ_i, λ_i+2are channels adjacent to λ_i+1. N is the total number of channels propagating in a span.</li></ul></li></ul>

p-0056In some embodiments, there may also be rules to determine when ghost channels <b>302</b> may not be generated. For example, if traffic channel <b>304</b> (denoted by λ_i) falls within the rule described below, then it may not be sufficiently isolated to warrant the generation of ghost channels <b>302</b>. <ul><li id="ul0005-0001" num="0000"><ul><li id="ul0006-0001" num="0063">if . . . n( . . . , λ_i−1, λ_i, λ_i+1, . . . )≧3 . . . and ( . . . , λ_i−1, λ_i, λ_i+1, . . . )ε{ch<b>1</b>, . . . , ch<b>44</b>}, and N={3 . . . , 44}</li><li id="ul0006-0002" num="0064">. . . , λ_i−1, λ_i, λ_i+1, . . . are neighbouring channels, n is the number of neighbouring channels, and N is the total number of channels propagating in a span.</li></ul></li></ul>

p-0057<figref idrefs="DRAWINGS">FIG. 4</figref> illustrates an optical amplification scheme <b>400</b> for generating ghost channels <b>302</b>, in accordance with certain embodiments of the present disclosure. In the illustrated embodiment, traffic channel <b>304</b> and ghost channels <b>302</b> are shown at specific amplitudes and separations. These and other specific properties of traffic channel <b>304</b> and ghost channels <b>302</b> are intended for illustrative purposes only, and are in no way intended to limit the scope of the present disclosure. When traffic channel <b>304</b> is amplified, e.g., by amplifier <b>206</b> of node <b>22</b>, a certain amount of noise is introduced through a phenomenon known as spontaneous emission. The amplification of traffic channel <b>304</b> may also amplify this noise, resulting in amplified spontaneous emission (ASE), an undesirable and problematic noise source, particularly for long-haul systems where traffic channel <b>304</b> may be amplified multiple times along its path. ASE is typically absorbed or extracted from network system <b>10</b> in order to maintain an acceptable signal to noise ratio.

p-0058However, controlling the location and magnitude of ASE may provide a source for the generation of ghost channels. Referring back to <figref idrefs="DRAWINGS">FIG. 4</figref>, four nodes <b>22</b> of network system <b>10</b> are shown, labeled as nodes <b>22</b><i>a</i>, <b>22</b><i>b</i>, <b>22</b><i>c</i>, and <b>22</b><i>d</i>. These labels are intended for clarity of discussion and are in no way intended to limit the scope of this disclosure. A traffic channel <b>304</b> may be added at node <b>22</b><i>a</i>. At node <b>22</b><i>a</i>, ASE at all incoming channels may be blocked, as shown in loss diagram <b>402</b><i>a</i>. Power spectrum <b>404</b><i>a </i>demonstrates that, as traffic channel <b>304</b> is transmitted from node <b>22</b><i>a </i>to node <b>22</b><i>b</i>, only traffic channel <b>304</b> is transmitted. Loss diagrams <b>402</b> and power spectrums <b>404</b> are provided for illustrative purposes only. As an example, in some embodiments, there may be multiple traffic channels <b>304</b> at varying separations.

p-0059When traffic channel <b>304</b> is amplified at node <b>22</b><i>b</i>, the ASE in the channels neighboring traffic channel <b>304</b> are not absorbed or extracted. The ASE in these neighboring channels may be allowed to grow to a certain point in order to provide the appropriate ghost channels <b>302</b> surrounding traffic channel <b>304</b>. Loss diagram <b>402</b><i>b </i>shows that the block levels applied to the neighbor channels have been reduced to a substantially lower level. This may allow the ASE in the neighbor channels to grow, generating a ghost channel, as shown in power spectrum diagram <b>404</b><i>b. </i>

p-0060At this stage, ghost channels <b>302</b> may have a low optical strength relative to traffic channel <b>304</b>. As traffic channel <b>304</b> and ghost channels <b>302</b> are passed through a third node <b>22</b><i>c</i>, ghost channels <b>302</b> may be amplified to a greater optical strength. Loss diagram <b>402</b><i>c </i>shows that the block levels applied to the neighbor channels are at the same substantially lower level as those in loss diagram <b>402</b><i>b</i>. This may allow the ASE in the neighbor channels to continue to grow, allowing ghost channels <b>302</b> to gain optical power, as shown in power spectrum diagram <b>404</b><i>c. </i>

p-0061This amplification may continue through a fourth node <b>22</b><i>d </i>until the optical strength of ghost channels <b>302</b> is at or near the optical strength of traffic channel <b>304</b>. At this stage ghost channels <b>302</b> may be at their most efficient in mitigating the effects of polarization hole burning without overcoming traffic channel <b>304</b>. Loss diagram <b>402</b><i>d </i>shows that the block levels applied to the neighbor channels have been raised relative to loss diagram <b>402</b><i>c</i>, but are still at a substantially lower level than in loss diagram <b>402</b><i>a</i>. This may allow the ASE in the neighbor channels to be capped at a certain optical power level, allowing ghost channels <b>302</b> to propagate at an optical power level substantially equal to traffic channel <b>304</b>, as shown in power spectrum diagram <b>404</b><i>d. </i>

p-0062Although this figure depicts this process taking place over the course of four nodes <b>22</b><i>a</i>-<i>d</i>, a particular implementation may take more or fewer nodes to get ghost channels <b>302</b> to an appropriate optical strength.

p-0063In some embodiments, degree <b>200</b> of node <b>22</b> may amplify ghost channels <b>302</b>. As part of the amplification, degree <b>200</b> may also determine whether ghost channel <b>302</b> needs to be amplified, or if it is already of sufficient magnitude. For instance, in some embodiments, it may be desirable to cap amplification of a ghost channel <b>302</b> as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 5-13</figref>. In other embodiments, the amplification and measurement may be done by more or different components of network system <b>10</b>.

p-0064<figref idrefs="DRAWINGS">FIG. 5</figref> is a flowchart illustrating one embodiment of a method <b>500</b> of mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method <b>500</b> includes checking an incoming traffic channel <b>304</b> for the existence of neighboring ghost channels <b>302</b>, generating ghost channels <b>302</b> if necessary, and amplifying ghost channels <b>302</b> if necessary.

p-0065According to one embodiment, method <b>500</b> preferably begins at step <b>502</b>. Teachings of the present disclosure may be implemented in a variety of configurations of nodes <b>22</b> and network system <b>10</b>. As such, the preferred initialization point for method <b>500</b> and the order of steps <b>502</b>-<b>512</b> comprising method <b>500</b> may depend on the implementation chosen. As described in more detail above with reference to <figref idrefs="DRAWINGS">FIG. 1</figref>, node <b>22</b> may be associated with a number of degrees <b>200</b>, each receiving multiplexed wavelengths <b>40</b> from a different portion of network system <b>10</b>. Depending on the implementation chosen, method <b>500</b> may be performed on some, all, or none of degrees <b>200</b> of node <b>22</b>. Additionally, multiplexed wavelengths <b>40</b> may be a multi-channel signal which may be demultiplexed into its component channels. Depending on the implementation chosen, method <b>500</b> may be performed on some, all, or none of the traffic channels <b>304</b> of multiplexed wavelengths <b>40</b>.

p-0066At step <b>502</b>, degree <b>200</b> of node <b>22</b> receives multiplexed wavelengths <b>40</b>. After receiving multiplexed wavelengths <b>40</b>, method <b>500</b> may begin to analyze a first channel constituting multiplexed wavelengths <b>40</b>. After analyzing that channel, method <b>500</b> may proceed to step <b>503</b>, where node <b>22</b> may determine whether the channel under consideration is a traffic channel <b>304</b>. In some embodiments, determining whether a given channel is a traffic channel <b>304</b> may constitute examining the WCS and WCF bits for that channel, as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>.

p-0067If the channel is not a traffic channel <b>304</b>, then method <b>500</b> may proceed to step <b>512</b>, where method <b>500</b> may proceed to examine the next channel before returning to step <b>502</b>. If the channel under consideration is a traffic channel <b>304</b>, then method <b>500</b> may proceed to step <b>504</b>, where node <b>22</b> may determine whether there are existing ghost channels <b>302</b> for traffic channel <b>304</b>. In some embodiments, step <b>504</b> may be performed by software controlling wavelength selection switch <b>204</b> of node <b>22</b>, or any other appropriately configured measurement module, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 1-4</figref>. In other embodiments, step <b>504</b> may be performed by hardware, firmware, or any other software module, including the operating system controlling node <b>22</b> configured to determine the presence of ghost channels <b>302</b>.

p-0068If no ghost channels <b>302</b> are currently present, method <b>500</b> may proceed to step <b>506</b>, where node <b>22</b> may generate ghost channels <b>302</b>. In some embodiments, step <b>506</b> may be performed by wavelength selection switch <b>204</b> of node <b>22</b>, or any other ghost channel generation module configured to modify the blocking level of ASE in appropriate channels, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 1-4</figref>. After generating ghost channels <b>302</b>, method <b>500</b> may proceed to step <b>508</b>. If, in step <b>504</b>, method <b>500</b> determined that there were extant ghost channels <b>302</b>, method <b>500</b> may proceed directly to step <b>508</b>.

p-0069At step <b>508</b>, method <b>500</b> may determine the power of ghost channels <b>302</b>, as measured by optical channel monitor <b>208</b> or any other appropriately configured power monitor, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 1-4</figref>. Method <b>500</b> may then compare the power of ghost channels <b>302</b> to the power of the associated traffic channel <b>304</b>. In some embodiments, this comparison may be performed by wavelength selection switch <b>204</b> of node <b>22</b>, or any other appropriately configured comparator, as described in more detail above with reference to FIGS. <b>1</b>-<b>4</b>. If ghost channels <b>302</b> are not of a sufficient magnitude, then method <b>500</b> may proceed to step <b>510</b>, where ghost channels <b>302</b> are amplified, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 1-4</figref>. After amplification, method <b>500</b> may proceed to step <b>512</b>.

p-0070If, in step <b>508</b>, method <b>500</b> determined that extant ghost channels <b>302</b> were already of a sufficient magnitude, then method <b>500</b> may proceed directly to step <b>512</b>. At step <b>512</b>, method <b>500</b> may proceed to examine the next channel before returning to step <b>502</b>.

p-0071Although <figref idrefs="DRAWINGS">FIG. 5</figref> discloses a particular number of steps to be taken with respect to method <b>500</b>, method <b>500</b> may be executed with more or fewer steps than those depicted in <figref idrefs="DRAWINGS">FIG. 5</figref>. In addition, although <figref idrefs="DRAWINGS">FIG. 5</figref> discloses a certain order of steps comprising method <b>500</b>, the steps comprising method <b>500</b> may be completed in any suitable order. For example, in the embodiment of method <b>500</b> shown, node <b>22</b> determines whether ghost channels <b>302</b> are substantially equal in magnitude to traffic channel <b>304</b>. However, in short-distance systems where design considerations may not require concern over potential overgrowth of ghost channels, these steps may not be necessary. Additionally, method <b>500</b> may also include additional steps concerning the determination of how close a ghost channel <b>302</b> may be in order to decide if additional ghost channels <b>302</b> may be necessary.

p-0072Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that using ghost channels <b>302</b> to mitigate the effects of polarization hole burning allows for a more robust solution to PHB effects in dynamically loaded optical communication network systems <b>10</b>. Another advantage may be that, since the methods and systems disclosed herein may be implemented in pre-existing hardware and/or software, the implementation costs and difficulties may be substantially reduced.

p-0073Effectively mitigating the effects of polarization hole burning in an optical communication system may require systems and/or methods of effectively managing, selecting, and/or optimizing the generated ghost channels <b>302</b>. Referring again to <figref idrefs="DRAWINGS">FIG. 2</figref>, in order to effectively manage ghost channel generation, it may be desirable to have the degrees <b>200</b> of node <b>22</b> communicate with one another. Intranodal communication may be used for many purposes, including the direction of incoming traffic over different output paths. Intranodal communication may also be used for communicating the current state of particular channels passing through each degree <b>200</b> of node <b>22</b>. This information can be important in the management of ghost channels <b>302</b> used for polarization hole burning mitigation.

p-0074In some embodiments, a degree <b>200</b> may need to add a ghost channel <b>302</b>, or pass-through (and potentially amplify) an existing ghost channel <b>302</b> from another degree <b>200</b> of node <b>22</b>. In order to effectively balance the load of the ghost channels across node <b>22</b>, it may be important to know the current and potential sources for ghost channels. However, the information required to effectively manage the ghost channel sources may not be readily available at each degree <b>200</b> of node <b>22</b>. For instance, the splitter input power for a given channel at Degree <b>1</b> may not be available to WSS <b>204</b> of Degree <b>4</b>. Without this information, Degree <b>4</b> may be unable to correctly determine whether to source a ghost channel from that given channel. In order to overcome these obstacles, degrees <b>200</b> may share information.

p-0075In some embodiments, degree <b>200</b> may collect certain pieces of information regarding the channels incoming to that degree <b>200</b>. That information may include a wavelength channel signal bit (“WCS”), a wavelength channel failure indicator (“WCF”), and the splitter input power for each channel. The WCS and WCF bits may be collected by amplifier card <b>212</b> of degree <b>200</b> and splitter input power may be collected by OCM <b>208</b> of degree <b>200</b>. However, these functions may be performed by the same component, different components, or any appropriately configured channel information module.

p-0076The WCS bit may be used to indicate whether a particular wavelength is intended to be present, e.g., whether information is being sent through that channel. In some embodiments, a “1” may indicate that a wavelength is intended to be present and a “0” may indicate that a wavelength is not intended to be present. The WCF bit may be used to indicate whether a wavelength is actually present. Although a wavelength may be intended to be present, some failure may have occurred. In some embodiments, a “1” may indicate that actual light is present, while a “0” may indicate that no light is present. These indicators, or others like them, may work together to indicate that a particular channel is “valid” for the purposes of ghost channel sourcing. In the illustrated configuration, a valid channel is one that is both intended to carry information and actually carrying information. However, other configuration may be based on different design decisions and define a valid channel differently without departing from the scope of this disclosure.

p-0077In some embodiments, each degree <b>200</b> may assemble a table for the desired channel validity information, with an entry for each incoming channel. These tables are discussed below in further detail with reference to <figref idrefs="DRAWINGS">FIGS. 5-9</figref>. Each degree <b>200</b> may then transmit these tables to every other degree <b>200</b> of node <b>22</b> in order to maximize information sharing and subsequent decision-making.

p-0078<figref idrefs="DRAWINGS">FIG. 6</figref> illustrates a table <b>600</b> for storing information regarding the validity of a channel incoming to a degree <b>200</b> of node <b>22</b>, in accordance with certain embodiments of the present disclosure. Table <b>600</b> may comprise a plurality of entries <b>602</b>, with each entry <b>602</b> corresponding to an incoming channel. For each entry <b>602</b> table <b>600</b> may store one or more value(s) <b>604</b>. In some embodiments, values <b>604</b> are the WCS and WCF values for each incoming channel. WCS and WCF values are discussed in more detail above with reference to <figref idrefs="DRAWINGS">FIG. 3</figref>. As an illustrative example, entry <b>602</b> for channel <b>1</b> may have a value of “1” for WCS and “0” for WCF. This may indicate that channel <b>1</b> is intended to be operational, but that no light is present. Entry <b>602</b> for channel <b>2</b> may have a “1” for WCS, indicating that it is intended to be operational, and a “1” for WCF, indicating that light is, in fact, present. The channel validity information of table <b>600</b> may then be passed on to a portion of degree <b>200</b> configured to measure optical strength of incoming channels. In some embodiments, the gathering of data in table <b>600</b> is performed by amplifier card <b>212</b> within degree <b>200</b> of node <b>22</b>. Amplifier card <b>212</b> may then send table <b>600</b> to switch card <b>214</b> in order to gather information regarding the optical power of the incoming channels as described in more detail below with reference to <figref idrefs="DRAWINGS">FIGS. 7-9</figref>. However, in other embodiments, table <b>600</b> may be generated within switch card <b>214</b> of degree <b>200</b> or by any channel information module of node <b>22</b> configured to gather the appropriate channel validity information.

p-0079<figref idrefs="DRAWINGS">FIG. 7</figref> illustrates a table <b>700</b> for storing information regarding the validity and signal strength of a channel incoming to a particular degree <b>200</b> of node <b>22</b>, in accordance with certain embodiments of the present disclosure. Table <b>700</b> may comprise a plurality of entries <b>702</b>, with each entry <b>702</b> corresponding to an incoming channel. For each entry <b>702</b>, table <b>700</b> may store one or more value(s) <b>704</b>. In some embodiments, values <b>704</b> are the WCS, WCF, and splitter input power for each incoming channel. WCS and WCF values are discussed in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 2 and 6</figref>. OCM <b>208</b> of degree <b>200</b> may measure the splitter input power for each channel and record the information in table <b>700</b>. Table <b>700</b> may then be broadcast to every other degree <b>200</b> of node <b>22</b>.

p-0080In some embodiments, switch card <b>214</b> of degree <b>200</b> receives channel validity information from amplifier card <b>212</b> (or some other channel information module), as described in more detail above with reference to <figref idrefs="DRAWINGS">FIG. 6</figref> (e.g., in the form of table <b>600</b>) and appends to that information regarding the optical power of the incoming channels from OCM <b>208</b> (e.g., in the form of splitter input power in table <b>700</b>). However, in other embodiments, these functions may be performed by the same component. For instance, the channel information module and switch card <b>214</b> may be an integral component configured to determine WCS bits, WCF bits, and to measure splitter input power at the same time without the need for separate tables <b>600</b> and <b>700</b>.

p-0081Once the information illustrated in table <b>700</b> has been broadcast to other degrees <b>200</b> of node <b>22</b>, it may be necessary or efficient for each degree <b>200</b> to consolidate the tables <b>700</b> received from each of the other degrees <b>200</b>.

p-0082<figref idrefs="DRAWINGS">FIG. 8</figref> illustrates a series of tables <b>800</b> representing the consolidated information received from other degrees <b>200</b> of node <b>22</b>, in accordance with certain embodiments of the present disclosure. In some embodiments, a degree <b>200</b> of node <b>22</b> may receive tables from other degrees <b>200</b> of node <b>22</b> containing information regarding the validity and optical power of certain channels, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-7</figref>. Aggregating such data in a way such that the data is available for each channel may be desirable, such as is depicted in table(s) <b>800</b>. It may also be desirable to include “freshness” data to indicate how recently the data has been received.

p-0083Each table <b>800</b> may represent the aggregated information for a particular channel. In some embodiments, there is a table <b>800</b> corresponding to each incoming channel (as an example only, the illustrated embodiment includes eighty-eight channels, so there are eighty-eight tables <b>800</b>). Each table <b>800</b> may include a plurality of entries <b>802</b>, with each entry <b>802</b> corresponding to a degree <b>200</b> in node <b>22</b>. Some configurations may also find it more desirable or efficient to combine one or more table(s) <b>800</b> into a single table <b>800</b>, or to split a single table <b>800</b> into smaller tables.

p-0084For each entry <b>802</b> table <b>800</b> may store one or more value(s) <b>804</b>. In some embodiments, values <b>804</b> are the WCS bit, WCF bit, splitter input power, and freshness values for each degree. WCS, WCF, and splitter input power values are described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-7</figref>. The “Fresh” value of table <b>800</b> may be used to indicate whether the remaining values <b>804</b> of table <b>800</b>, have been recently updated (or sufficiently “fresh”). In some embodiments, switch card <b>214</b> of degree <b>200</b> may determine whether the data stored in table <b>800</b> has been refreshed within a predetermined amount of time, e.g., five seconds. If the data has been received within that time period, then the Fresh value <b>804</b> may be marked with a “1” to indicate that the data is sufficiently fresh; if is has not, then it may be marked with a “0” to indicate that the data is stale.

p-0085In addition to information regarding the validity and freshness of the incoming channels at each degree <b>200</b> of node <b>22</b>, it may also be desirable to know what, if any, ghost channels are already sourced from a degree <b>200</b>.

p-0086<figref idrefs="DRAWINGS">FIG. 9</figref> illustrates a table <b>900</b> for storing information regarding the current ghost channel load for degrees <b>200</b> in node <b>22</b>, in accordance with certain embodiments of the present disclosure. Table <b>900</b> may include a plurality of entries <b>902</b>, with each entry <b>902</b> corresponding to a degree <b>200</b> in node <b>22</b>. For each entry <b>902</b> table <b>900</b> may store one or more value(s) <b>904</b>. In some embodiments, value <b>904</b> is the current ghost channel count for each degree <b>200</b> in node <b>22</b>. The ghost channel count is the number of ghost channels that are currently sourced from a particular degree. The data in current ghost count <b>904</b> may be used to efficiently balance the ghost channel load within node <b>22</b>. This information may allow the distribution of ghost channels across degrees <b>200</b> within node <b>22</b>, which may in turn improve operational efficiency of node <b>22</b> and network system <b>10</b>. Table <b>900</b> may, in some embodiments, be stored and managed by the software code that operates WSS <b>204</b> of degree <b>200</b>. In other embodiments, table <b>900</b> may be managed by an overarching node management system, and implemented in software, hardware, or firmware, or some combination thereof. The information in table <b>900</b> regarding the current ghost channel load of degrees <b>200</b> may be combined with the channel input information in table <b>600</b> to better manage the ghost channels used to mitigate the effects of polarization hole burning.

p-0087<figref idrefs="DRAWINGS">FIG. 10</figref> is a flowchart illustrating one embodiment of a method <b>1000</b> of managing the selection of ghost channels in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method <b>1000</b> includes collecting validity and optical power data for optical communication channels, transmitting that data to all degrees <b>200</b> within a node <b>22</b>, aggregating received data, and collecting data concerning the current ghost channel loading.

p-0088According to one embodiment, method <b>1000</b> preferably begins at step <b>1002</b>. Teachings of the present disclosure may be implemented in a variety of configurations of node and network system <b>10</b>. As such, the preferred initialization point for method <b>1000</b> and the order of steps <b>1002</b>-<b>1010</b> comprising method <b>1000</b> may depend on the implementation chosen.

p-0089At step <b>1002</b>, method <b>1000</b> collects validity data for an optical communication channel at a first degree <b>200</b> (as an example only, the WCS and WCF bits for the optical communication channel). In some embodiments, this step may be performed by a channel information module as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-9</figref>. After collecting this information method <b>1000</b> may proceed to step <b>1004</b>.

p-0090At step <b>1004</b>, method <b>1000</b> collects optical power data for the optical communication channel at each degree <b>200</b>. In some embodiments, step <b>1004</b> may be performed by OCM <b>208</b> as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-9</figref>. After collecting this information method <b>1000</b> may proceed to step <b>1006</b>.

p-0091At step <b>1006</b>, method <b>1000</b> may transmit all of the collected validity and optical power data to all other degrees <b>200</b> within node <b>22</b>. Step <b>1006</b> may be performed by switch card <b>214</b> as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-9</figref>. After transmitting this data, method <b>1000</b> may proceed to step <b>10101008</b>.

p-0092At step <b>1008</b>, method <b>1000</b> may aggregate the channel validity and optical power data received at each degree <b>200</b> from all other degrees <b>200</b> such that a composite picture of the data for a given channel may be formed. The aggregated validity and power data may be combined with a freshness value, indicating how recently the data had been retrieved. Step <b>1008</b> may be performed by switch card <b>214</b> as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-9</figref>. After aggregating and collecting this data, method <b>1000</b> may proceed to step <b>1010</b>.

p-0093At step <b>1010</b>, method <b>1000</b> may collect data regarding the current ghost channel load within node <b>22</b>. This data may comprise information detailing which degrees <b>200</b> within node <b>22</b> are currently being used as ghost channel sources, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIG. 9</figref>. After collecting this data, method <b>1000</b> may return to step <b>1002</b> to begin the data collection cycle again. In some embodiments, there may be a time delay, such as five seconds, before the cycle begins again. Such a time delay would be a design determination to suit the particular implementation of network system <b>10</b>.

p-0094Although <figref idrefs="DRAWINGS">FIG. 10</figref> discloses a particular number of steps to be taken with respect to method <b>1000</b>, method <b>1000</b> may be executed with more or fewer steps than those depicted in <figref idrefs="DRAWINGS">FIG. 10</figref>. In addition, although <figref idrefs="DRAWINGS">FIG. 10</figref> discloses a certain order of steps comprising method <b>1000</b>, the steps comprising method <b>1000</b> may be completed in any suitable order. For example, in the embodiment of method <b>1000</b> shown, degree <b>200</b> of node <b>22</b> collects channel validity data and optical power data in two separate steps. In some embodiments, it may be desirable to separate these steps to have separate components of degree <b>200</b> perform these tasks. However, in other embodiments these steps may be performed simultaneously and/or by the same component of degree <b>200</b>.

p-0095Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that effective management of ghost channel selection can allow a network system to effectively balance the load that ghost channels may place on the system.

p-0096Along with managing the load balance of ghost channels <b>302</b> within node <b>22</b>, it may also be beneficial to select the appropriate source for ghost channels <b>302</b> in order to reduce or eliminate the potential deleterious effects of feedback when mitigating the effects of polarization hole burning.

p-0097<figref idrefs="DRAWINGS">FIG. 11</figref> is a flowchart illustrating one embodiment of a method <b>1100</b> of selecting ghost channels for use in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method <b>1100</b> includes checking each channel to see if it is a traffic channel, a blocked channel, a current ghost channel, or if its neighbor channels are add or pass-through channels. In operation, method <b>1100</b> determines whether, for a given channel, either neighbor channel will have a signal when it leaves degree <b>200</b>. If it does, then the current channel may need to be blocked from use as a ghost channel in order to prevent undesirable feedback within network system <b>10</b>. One point at which it may be effective to reduce feedback is at the point where a traffic channel is added. If the traffic channel is being added at node <b>22</b>, then ghost channel <b>302</b> may not be immediately introduced at node <b>22</b>. If a traffic channel is a pass through channel, then ghost channel <b>302</b> may be generated with less concern for feedback.

p-0098According to one embodiment, method <b>1100</b> preferably begins at step <b>1102</b>. Teachings of the present disclosure may be implemented in a variety of configurations of communication system <b>10</b>. As such, the preferred initialization point for method <b>1100</b> and the order of steps <b>1102</b>-<b>1116</b> comprising method <b>1100</b> may depend on the implementation chosen. In some embodiments, the steps of method <b>1100</b> may be performed by the software that manages WSS <b>204</b> of each degree <b>200</b>. In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within each degree <b>200</b>.

p-0099Beginning with a first channel, at step <b>1102</b>, method <b>1100</b> determines whether the first channel is a traffic channel. In some embodiments, determining whether a given channel is a traffic channel <b>304</b> may constitute examining the WCS and WCF bits for that channel, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>. If the channel is a traffic channel, then method <b>1100</b> may proceed to step <b>1108</b>, wherein method <b>1100</b> advances to the next channel before returning to step <b>1102</b>. If the channel is not a traffic channel, then method <b>1100</b> may proceed to step <b>1104</b>.

p-0100At step <b>1104</b>, method <b>1100</b> determines whether either channel neighboring the channel under consideration is an add channel. In some embodiments, an add channel is a channel on which traffic is added at WSS <b>204</b> of the degree <b>200</b> performing the method. In such a situation, sourcing a ghost channel may lead to undesirable feedback within network system <b>10</b>. If either neighbor channel is an add channel, then method <b>1100</b> may proceed to step <b>1106</b>. If neither neighbor channel is an add channel, then method <b>1100</b> may proceed to step <b>1112</b>.

p-0101At step <b>1106</b> method <b>1100</b> may determine whether the channel under consideration is currently blocked. If it is blocked, then method <b>1100</b> may proceed to step <b>1108</b>, wherein method <b>1100</b> advances to the next channel before returning to step <b>1102</b>. If the current channel under consideration is not blocked, then method <b>1100</b> may proceed to step <b>1110</b>, wherein the channel is blocked, before proceeding to step <b>1108</b>.

p-0102At step <b>1112</b>, method <b>1100</b> may determine whether either neighbor channel is a pass-through channel. In some embodiments, a pass-through channel is a channel on which traffic is received at the degree <b>200</b> and which is passed through WSS <b>204</b> of that degree <b>200</b>. In the case of pass-through channels, sourcing a ghost channel from the channel under consideration may not run as high a risk of undesirable feedback within network system <b>10</b>. If either neighbor channel is a pass-through channel, then method <b>1100</b> may proceed to step <b>1114</b>. If neither neighbor channel is a pass-through channel, then method <b>1100</b> may proceed to step <b>1106</b>.

p-0103At step <b>1114</b>, method <b>1100</b> may determine whether the current channel is already being used to source a ghost channel. If it is, then method <b>1100</b> may proceed to step <b>1108</b>, wherein method <b>1100</b> advances to the next channel before returning to step <b>1102</b>. If it is not, then method <b>1100</b> may proceed to step <b>1116</b>. At step <b>1116</b>, method <b>1100</b> may select the current channel to source a ghost channel. The selection method may be simple or complex, depending on the particular implementation. In some embodiments, the ghost source may be selected from a calculated set of degrees with appropriate validity and optical power criteria, as described below in more detail with reference to <figref idrefs="DRAWINGS">FIG. 12</figref>.

p-0104Although <figref idrefs="DRAWINGS">FIG. 11</figref> discloses a particular number of steps to be taken with respect to method <b>1100</b>, method <b>1100</b> may be executed with more or fewer steps than those depicted in <figref idrefs="DRAWINGS">FIG. 11</figref>. In addition, although <figref idrefs="DRAWINGS">FIG. 11</figref> discloses a certain order of steps comprising method <b>1100</b>, the steps comprising method <b>1100</b> may be completed in any suitable order. For example, in the embodiment of method <b>1100</b> shown, channels with neighbors that will carry add channels after leaving degree <b>200</b> may be blocked. However, in some configurations it may be less important to guard against feedback and these steps may be reduced in scope or eliminated.

p-0105<figref idrefs="DRAWINGS">FIG. 12</figref> is a flowchart illustrating one embodiment of a method <b>1200</b> of selecting the degree <b>200</b> of node <b>22</b> from which to select the appropriate ghost channels for mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method <b>1200</b> includes selecting the source degree <b>200</b> for the ghost channel <b>302</b> identified above with reference to <figref idrefs="DRAWINGS">FIG. 11</figref> by examining the validity and optical power values of channels within the degrees <b>200</b> of node <b>22</b>, in order to maintain a desired level of load balancing across network system <b>10</b>.

p-0106In operation, method <b>1200</b> may select the degree <b>200</b> of node <b>22</b> that can source a ghost channel <b>302</b> with the highest initial optical power (e.g., the ASE noise at the highest initial level) and/or the degree with the lowest current ghost channel count.

p-0107According to one embodiment, method <b>1200</b> preferably begins at step <b>1202</b>. Teachings of the present disclosure may be implemented in a variety of configurations of communication system <b>10</b>. As such, the preferred initialization point for method <b>1200</b> and the order of steps <b>1202</b>-<b>1220</b> comprising method <b>1200</b> may depend on the implementation chosen. In some embodiments, the steps of method <b>1200</b> may be performed by the software that manages WSS <b>204</b> of degree <b>200</b>. In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within degree <b>200</b>.

p-0108At step <b>1202</b>, method <b>1200</b> may determine whether a given channel is valid, fresh, and has a splitter input power greater than or equal to a predetermined threshold. Channel validity, freshness, and optical power information are discussed in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>. The predetermined threshold may be any value determined by the particular implementation of network system <b>10</b> to produce a likelihood of finding an appropriate ghost channel while maintaining the balance of ghost channel load across degrees <b>200</b> of node <b>22</b>. The set of degrees <b>200</b> meeting both criteria is depicted in method <b>1200</b> by the letter “D.” After examining each degree <b>200</b>, the method may proceed to step <b>1204</b>.

p-0109At step <b>1204</b>, method <b>1200</b> may determine whether there are any degrees <b>200</b> within the set that met both criteria in step <b>1202</b> (e.g., whether D=0). If there are any sufficiently valid, fresh, and powerful channels in any degree <b>200</b> (D does not equal 0), then method <b>1200</b> may proceed to step <b>1206</b>. If there are not, then method <b>1200</b> may proceed to step <b>1212</b>.

p-0110At step <b>1206</b>, method <b>1200</b> may determine which degree currently has the least number of ghost channels currently sourced. The gathering, collecting, and transmission of current ghost channel load information is described above in more detail with reference to <figref idrefs="DRAWINGS">FIGS. 6-10</figref>. In <figref idrefs="DRAWINGS">FIG. 12</figref>, this degree is denoted by the letter “d.” After determining the appropriate degree, method <b>1200</b> may proceed to step <b>1208</b>. At step <b>1208</b>, the controller may switch in the designated ghost channel from the designated degree d. Method <b>1200</b> may then proceed to step <b>1210</b>, where the ghost channel count for the designated degree is incremented, at which point method <b>1200</b> may proceed to step <b>1220</b>, where method <b>1200</b> terminates.

p-0111If there are no sufficiently valid, fresh, powerful channels found at step <b>1204</b> (e.g., D=0), then method <b>1200</b> may proceed to step <b>1212</b>, where method <b>1200</b> may limit the determination to the number of sufficiently valid and fresh channels (denoted by the letter “D.”). After determining the number of valid and fresh channels, method <b>1200</b> may proceed to step <b>1214</b>. At step <b>1214</b>, the controller may determine if there were any valid and fresh channels found in step <b>1212</b>. If there were no valid and fresh channels (D=0), then method <b>1200</b> may proceed to step <b>1216</b>, where the channel under examination is blocked as a ghost source. After blocking the channel, method <b>1200</b> may proceed to step <b>1220</b>, where method <b>1200</b> terminates.

p-0112If, in step <b>1214</b>, there were one or more valid and fresh channels, then method <b>1200</b> may proceed to step <b>1218</b>. At step <b>1218</b>, method <b>1200</b> may determine which degree has the channel with the highest splitter input power. This degree is denoted by the letter “d” in the drawing. After making this determination, method <b>1200</b> may proceed to step <b>1208</b>. At step <b>1208</b>, method <b>1200</b> may switch in the ghost channel from the designated degree. As discussed above, after switching in the ghost channel, method <b>1200</b> may then proceed to step <b>1210</b>, where the ghost count is incremented for the designated degree, and then to step <b>1220</b>, where method <b>1200</b> terminates.

p-0113Although <figref idrefs="DRAWINGS">FIG. 12</figref> discloses a particular number of steps to be taken with respect to method <b>1200</b>, method <b>1200</b> may be executed with more or fewer steps than those depicted in <figref idrefs="DRAWINGS">FIG. 12</figref>. In addition, although <figref idrefs="DRAWINGS">FIG. 12</figref> discloses a certain order of steps comprising method <b>1200</b>, the steps comprising method <b>1200</b> may be completed in any suitable order. For example, in the embodiment of method <b>1200</b> shown, a choice may be made to switch in a channel with a splitter input of less than the full ghost threshold. However, in some configurations it may be desirable to only source ghost channels from channels with a splitter input greater than or equal to the full ghost threshold.

p-0114In order to maintain the effectiveness of a load balanced, ghost channel polarization hole burning mitigation scheme, it may be necessary or desirable to optimize that scheme for continued performance.

p-0115<figref idrefs="DRAWINGS">FIG. 13</figref> is a flowchart illustrating one embodiment of a method <b>1300</b> of optimizing a ghost channel selection routine in order to mitigate the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method <b>1300</b> includes periodically running a ghost channel selection algorithm in order to ensure that the most effective degree <b>200</b> of node <b>22</b> is the current source for a ghost channel <b>302</b>.

p-0116According to one embodiment, method <b>1300</b> preferably begins at step <b>1302</b>. Teachings of the present disclosure may be implemented in a variety of configurations of communication system <b>10</b>. As such, the preferred initialization point for method <b>1300</b> and the order of steps <b>1302</b>-<b>1308</b> comprising method <b>1300</b> may depend on the implementation chosen. In some embodiments, the steps of method <b>1300</b> may be performed by the software that manages WSS <b>204</b> of degree <b>200</b>. In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within degree <b>200</b>.

p-0117At step <b>1302</b>, method <b>1300</b> may run a ghost selection routine for a first channel. The ghost selection routine may be simple or complex, depending on the configuration of network system <b>10</b>. In some embodiments, the ghost source may be selected from a calculated set of degrees with appropriate validity and optical power criteria, as described in more detail above with reference to <figref idrefs="DRAWINGS">FIGS. 11-12</figref>. Once the ghost selection routine has run for the first channel, method <b>1300</b> may proceed to step <b>1304</b>.

p-0118At step <b>1304</b>, method <b>1300</b> may determine the current source degree for the ghost channel. Method <b>1300</b> may then proceed to step <b>1305</b>. At step <b>1305</b>, method <b>1300</b> may determine whether the ghost channel source degree returned from the selection routine is different from the current ghost channel source degree. If it is not, then method <b>1300</b> may proceed to step <b>1308</b>, wherein method <b>1300</b> may proceed to the next ghost channel and repeat the method by returning to step <b>1302</b>. If the ghost channel selection routine returns a different source than what is currently used, then method <b>1300</b> may proceed to step <b>1306</b>. At step <b>1306</b>, method <b>1300</b> may switch the source of the current ghost channel from the degree <b>200</b> currently being used to the degree <b>200</b> returned from the ghost channel selection routine in step <b>1302</b>. After switching the ghost channel source, method <b>1300</b> may proceed to step <b>1308</b>, wherein method <b>1300</b> may proceed to the next ghost channel and repeat the method by returning to step <b>1302</b>.

p-0119Although <figref idrefs="DRAWINGS">FIG. 13</figref> discloses a particular number of steps to be taken with respect to method <b>1300</b>, method <b>1300</b> may be executed with more or fewer steps than those depicted in <figref idrefs="DRAWINGS">FIG. 13</figref>. In addition, although <figref idrefs="DRAWINGS">FIG. 13</figref> discloses a certain order of steps comprising method <b>1300</b>, the steps comprising method <b>1300</b> may be completed in any suitable order. For example, in the embodiment of method <b>1300</b> shown, the ghost channel selection routine is run for every ghost channel. However, in some configurations it may not be desirable or efficient to run the routine for every ghost channel continuously. It may be more efficient to, for instance, only run the routine for every other ghost channel the first time through method <b>1300</b>, and run the routine for the other half of ghost channels the second time through method <b>1300</b>.

p-0120While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.